1. Field of the Invention
The present invention relates to a mass spectrometer and a method of mass spectrometry.
2. Discussion of the Prior Art
Mass spectrometry is an established technique for identifying and quantifying molecules, including molecules of biological interest. It is a primary technique for identifying proteins due to its unparalleled speed, sensitivity and specificity. Strategies for the analysis of proteins may involve either analysis of the intact protein or more commonly digestion of the protein using a specific protease that cleaves at predictable residues along the peptide backbone. This provides smaller stretches of peptide sequence that are more amenable to analysis via mass spectrometry.
It is known to perform experiments which involve the separation of a complex digest mixture by liquid chromatography which is directly interfaced to a tandem mass spectrometer using Electrospray Ionisation (ESI). MS and MS/MS spectra may be collected throughout the chromatographic separation and this information may be used to search databases directly for matching sequences leading to identification of the parent protein.
The known approach can be used to identify proteins that are present at low endogenous concentrations. However, such digest mixtures may contain many hundreds if not thousands of components many of which will co-elute from the chromatography column. Methods designed for analysis of digest mixtures aim to identify as many of the peaks as possible within the complex mixture. However, as sample complexity increases it becomes increasingly difficult to select each individual precursor or parent ion for subsequent fragmentation.
One method of increasing the peak capacity is to fragment a large number of parent or precursor ions simultaneously and then to record their product or fragment ions. Product or fragment ions may be associated with parent or precursor ions according to the closeness of alignment of their LC elution times. Eventually, however, as the sample complexity increases this method may also fail.
Another approach to the problem of highly complex mixtures is to improve the separation capability. Addition of a further orthogonal separation stage can be particularly effective, especially if the time requirements for each separation process and for the mass spectrometer do not overlap.
One known method which may be used to separate ions prior to analysis by mass spectrometry is that of ion mobility spectrometry or gas phase electrophoresis. One form of an ion mobility spectrometer or separator comprises a drift tube or cell wherein an axial electric field is maintained in the presence of a buffer gas. Higher mobility ions pass more quickly along the length of the ion mobility spectrometer or separator than lower mobility ions. As a result ions are separated according to their ion mobility.
A known ion mobility spectrometer or separator may operate at or around atmospheric pressure or under a partial vacuum at a pressure down to as low as about 0.01 mbar. The known ion mobility spectrometer or separator operating under a partial vacuum comprises a plurality of electrodes having apertures. A DC voltage gradient is maintained along the length of the ion mobility spectrometer or separator and the electrodes are connected to an AC or RF voltage supply. This form of ion mobility spectrometer or separator is advantageous in that the AC or RF voltage which is applied to the electrodes results in radial confinement of the ions passing through the ion mobility spectrometer or separator. Radial confinement of the ions results in higher ion transmission compared with an ion mobility spectrometer or separator which does not confine ions radially.
An ion mobility spectrometer or separator is known wherein ions are confined radially by an inhomogeneous RF field in an ion guide and ions are propelled forward by a potential hill or barrier that is progressively applied along the axis of the ion guide in the presence of a buffer gas. Appropriate selection of the amplitude and velocity of the potential hill or barrier which is translated along the length of the ion guide and the type and pressure of gas allows ions to slip selectively over the potential hill or barrier according to their ion mobility. This in turn allows ions having different ion mobilities to be transported at different velocities along the ion guide and thereby to become temporally separated.
The additional separation gained by the use of ion mobility separation (IMS) or gas phase electrophoresis increases the peak capacity of a mass spectrometer. This benefit is gained irrespective of whether or not other separation techniques such as Liquid Chromatography (LC) are also used. Furthermore, the benefit gained by the use of ion mobility separation is equally relevant to tandem mass spectrometers (MS/MS) in which parent ions may be mass analysed and then selected parent ions may be induced to fragment by Collision Induced Decomposition and wherein the resulting fragment or daughter ions are then mass analysed.
It is desired to provide a mass spectrometer having an improved ability to separate ions according to their ion mobility.